Optoelectronic devices rely on the optical and electronic properties of materials to either produce or detect electromagnetic radiation or to generate electricity from ambient electromagnetic radiation. Photosensitive optoelectronic devices convert electromagnetic radiation into electricity. Solar cells, also known as photovoltaic (PV) devices, are used to generate electrical power from ambient light. PV devices are used to drive power consuming loads to provide, for example, lighting, heating, or to operate electronic equipment such as computers or remote monitoring or communications equipment. These power generation applications often involve the charging of batteries or other energy storage devices so that equipment operation may continue when direct illumination from the sun or other ambient light sources is not available. As used herein, the term “resistive load” refers to any power consuming or storing device, equipment or system.
Traditionally, photosensitive optoelectronic devices have been constructed of a number of inorganic semiconductors, e.g., crystalline, polycrystalline and amorphous silicon, gallium arsenide, cadmium telluride and others. Herein the term “semiconductor” denotes materials which can conduct electricity when charge carriers are induced by thermal or electromagnetic excitation. The term “photoconductive” generally relates to the process in which electromagnetic radiant energy is absorbed and thereby converted to excitation energy of electric charge carriers so that the carriers can conduct, i.e., transport, electric charge in a material. The terms “photoconductor” and “photoconductive material” are used herein to refer to semiconductor materials which are chosen for their property of absorbing electromagnetic radiation to generate electric charge carriers.
Solar cells are characterized by the efficiency with which they can convert incident solar power to useful electric power. Devices utilizing crystalline or amorphous silicon dominate commercial applications, and some have achieved efficiencies of 23% or greater. However, efficient crystalline-based devices, especially of large surface area, are difficult and expensive to produce due to the problems inherent in producing large crystals without significant efficiency-degrading defects. On the other hand, high efficiency amorphous silicon devices still suffer from problems with stability. Present commercially available amorphous silicon cells have stabilized efficiencies between 4 and 8%. More recent efforts have focused on the use of organic photovoltaic cells to achieve acceptable photovoltaic conversion efficiencies with economical production costs.
PV devices produce a photo-generated voltage when they are connected across a load and are irradiated by light. When irradiated without any external electronic load, a PV device generates its maximum possible voltage, V open-circuit, or VOC. If a PV device is irradiated with its electrical contacts shorted, a maximum short-circuit current, or ISC, is produced. When actually used to generate power, a PV device is connected to a finite resistive load and the power output is given by the product of the current and voltage, I×V. The maximum total power generated by a PV device is inherently incapable of exceeding the product, ISC×VOC. When the load value is optimized for maximum power extraction, the current and voltage have values, Imax and Vmax, respectively.
A figure of merit for solar cells is the fill factor, ff, defined as:
                    ff        =                                            I              max                        ⁢                          V              max                                                          I                              S                ⁢                                                                  ⁢                C                                      ⁢                          V                              O                ⁢                                                                  ⁢                C                                                                        (        1        )            where ff is always less than 1, as ISC and VOC are never obtained simultaneously in actual use. Nonetheless, as ff approaches 1, the device is more efficient.
When electromagnetic radiation of an appropriate energy is incident upon a semiconductive organic material, for example, an organic molecular crystal (OMC) material, or a polymer, a photon can be absorbed to produce an excited molecular state. This is represented symbolically as S0+hvS0*. Here S0 and S0* denote ground and excited molecular states, respectively. This energy absorption is associated with the promotion of an electron from a bound state in the HOMO, which may be a π-bond, to the LUMO, which may be a π*-bond, or equivalently, the promotion of a hole from the LUMO to the HOMO. In organic thin-film photoconductors, the generated molecular state is generally believed to be an exciton, i.e., an electron-hole pair in a bound state which is transported as a quasi-particle. The excitons can have an appreciable life-time before geminate recombination, which refers to the process of the original electron and hole recombining with each other, as opposed to recombination with holes or electrons from other pairs. To produce a photocurrent the electron-hole pair must become separated, typically at a donor-acceptor interface between two dissimilar contacting organic thin films. If the charges do not separate, they can recombine in a geminant recombination process, also known as quenching, either radiatively, by the emission of light of a lower energy than the incident light, or non-radiatively, by the production of heat. Either of these outcomes is undesirable in a photosensitive optoelectronic device.
Electric fields or inhomogeneities at a contact may cause an exciton to quench rather than dissociate at the donor-acceptor interface, resulting in no net contribution to the current. Therefore, it is desirable to keep photogenerated excitons away from the contacts. This has the effect of limiting the diffusion of excitons to the region near the junction so that the associated electric field has an increased opportunity to separate charge carriers liberated by the dissociation of the excitons near the junction.
To produce internally generated electric fields which occupy a substantial volume, the usual method is to juxtapose two layers of material with appropriately selected conductive properties, especially with respect to their distribution of molecular quantum energy states. The interface of these two materials is called a photovoltaic heterojunction. In traditional semiconductor theory, materials for forming PV heterojunctions have been denoted as generally being of either n, or donor, type or p, or acceptor, type. Here n-type denotes that the majority carrier type is the electron. This could be viewed as the material having many electrons in relatively free energy states. The p-type denotes that the majority carrier type is the hole. Such material has many holes in relatively free energy states. The type of the background, i.e., not photo-generated, majority carrier concentration depends primarily on unintentional doping by defects or impurities. The type and concentration of impurities determine the value of the Fermi energy, or level, within the gap between the highest occupied molecular orbital (HOMO) and the lowest unoccupied molecular orbital (LUMO), called the HOMO-LUMO gap. The Fermi energy characterizes the statistical occupation of molecular quantum energy states denoted by the value of energy for which the probability of occupation is equal to ½. A Fermi energy near the LUMO energy indicates that electrons are the predominant carrier. A Fermi energy near the HOMO energy indicates that holes are the predominant carrier. Accordingly, the Fermi energy is a primary characterizing property of traditional semiconductors and the prototypical PV heterojunction has traditionally been the p-n interface.
The term “rectifying” denotes, inter alia, that an interface has an asymmetric conduction characteristic, i.e., the interface supports electronic charge transport preferably in one direction. Rectification is associated normally with a built-in electric field which occurs at the heterojunction between appropriately selected materials.
A significant property in organic semiconductors is carrier mobility. Mobility measures the ease with which a charge carrier can move through a conducting material in response to an electric field. As opposed to free carrier concentrations, carrier mobility is determined in large part by intrinsic properties of the organic material such as crystal symmetry and periodicity. Appropriate symmetry and periodicity can produce higher quantum wavefunction overlap of HOMO levels producing higher hole mobility, or similarly, higher overlap of LUMO levels to produce higher electron mobility. Moreover, the donor or acceptor nature of an organic semiconductor, e.g., 3,4,9,10-perylenetetracarboxylic dianhydride (PTCDA), may be at odds with the higher carrier mobility. For example, while chemistry arguments suggest a donor, or n-type, character for PTCDA, experiments indicate that hole mobilities exceed electron mobilities by several orders of magnitude so that the hole mobility is a critical factor. The result is that device configuration predictions from donor/acceptor criteria may not be borne out by actual device performance. Due to these unique electronic properties of organic materials, rather than designating them as “p-type” or “acceptor-type” and “n-type” or “donor-type”, the nomenclature of “hole-transporting-layer” (HTL) or “electron-transporting-layer” (ETL) is frequently used. In this designation scheme, an ETL will be preferentially electron conducting and an HTL will be preferentially hole transporting.
A typical prior art photovoltaic device configuration is the organic bilayer cell. In the bilayer cell, charge separation predominantly occurs at the organic heterojunction. The built-in potential is determined by the HOMO-LUMO energy difference between the two materials contacting to form the heterojunction. The HOMO-LUMO gap offset between the HTL and ETL produce an electric field around the HTL/ETL interface.
Organic PV cells have many potential advantages when compared to traditional silicon-based devices. Organic PV cells are light weight, economical in materials use, and can be deposited on low cost substrates, such as flexible plastic foils. However, organic PV devices typically have relatively low quantum yield (the ratio of photons absorbed to carrier pairs generated, or electromagnetic radiation to electricity conversion efficiency), being on the order of 1% or less. This is, in part, thought to be due to the second order nature of the intrinsic photoconductive process. That is, carrier generation requires exciton generation, diffusion and ionization. However, the diffusion length (LD) of an exciton is typically much less (LD˜50 Å) than the optical absorption length (˜500 Å), requiring a trade off between using a thick, and therefore resistive, cell with multiple or highly folded interfaces, or a thin cell with a low optical absorption efficiency. Different approaches to increase the efficiency have been demonstrated, including use of doped organic single crystals, conjugated polymer blends, and use of materials with increased exciton diffusion length. The problem was attacked yet from another direction, namely employment of different cell geometry, such as three-layered cell, having an additional mixed layer of co-deposited p- and n-type pigments, or fabricating tandem cell.
As was shown in previous work, the open circuit voltage (VOC) of the tandem cell can be almost two times higher than that of the single cell. M. Hiramoto, M. Suezaki, and M. Yokoyama, Chemistry Letters, 327 (1990). Unfortunately, the obtained power conversion efficiency was less than for the single cell. That was attributed to the fact that the front cell attenuates light intensity coming to the back cell. Therefore the photocurrent produced by the back cell is reduced, thus limiting the power conversion efficiency. Additional reasoning was that doubling of the overall thickness in tandem devices results in increased series resistance, that also could affect the power conversion efficiency.
In order to increase the cell performance, materials and device configurations are desirable which can enhance the quantum yield and, therefore, the power conversion efficiency. We have now achieved high power conversion efficiency by fabrication of a photovoltaic cell comprising multiple stacked subcells with thickness optimization and employing an electron blocking layer.